Interference reducing circuit



Feb. 6, 1951 A. MILLER 2,540,954

INTEREFERENCE REDUCING CIRCUIT Filed Dec. 28, 1949 2 Sheets-Sheer?l l INTEREFERENCE REDUCING CIRCUIT Filed Dec. 28, 1949 2 sheets-sheet 2 A Mrunen F n rzn Sfcrlon @ri/vr Mller Patented Feb. 6, 1951 UNITED STATES PATENT OFFICE INTERFEBENCE REDUCING CIRCUIT Arthur Miller, Watertown, Mass., assignor to Sanborn Company, Cambridge, Mass., a corporation of Massachusetts Application December 28, 1949, Serial No. 135,355

4 Claims.

In many types of portable electrical equipment it is necessary in ordei` to reduce electrical interference to connect the frame or chassis to ground. lAs is described in my United States In a broad aspect the invention contemplates an interference reducing circuit for an amplifier comprising a transformer, which may be the transformer of the direct power supply to the Letters Patent No. 2,500,994 of which the present 5 amplier, having a secondary or tertiary winding application is a continuation in part, if the connected to the ampliiier chassis and a primary chassis comprising the return electrical path for winding each end of which is linked to a respecthe amplifying and recording circuits of an tive terminal for connecting the primary winding electrocardiograph is not carefully connected by to a grounded alternating power source. The a low impedance path to a water pipe or other chassis is coupled by means of an impedance, conductor at ground potential, a high alternat-A having either reactive or ohmic characteristics, ing potential may be developed between the to one of the terminals. The characteristics of chassis and ground. The patients body is conthe impedance are low enough with respect to nected to the input of the amplifier and acts as the stray capacitances of the transformer so that an antenna picking up stray alternating voltwith respect to the voltage introduced by such ages some portion of which may appear after capacitances the chassis is substantially at amplification in the iinal electrocardiograph ground potential, while being great enough to record so that the interpretation thereof is diiintroduce a voltage drop which will prevent a cult or impossible. If the chassis is at a high lethal shock upon bodily contact with the chassis potential as a result of the omission of the ground if the primary terminal coupled thereto is acciconnection when the patient is connected to the dentally connected to the ungrounded side of the chassis by the ampliiier input circuit, the instrualternating power source. ment chassis itself may constitute the greatest Another aspect of the invention concerns the source of interference voltage. introduction of an electrical network between the Energy to operate such amplifiers is usually primary winding terminal coupled to the chassis obtained from a commercial alternating power source one conductor of which is at ground potential. The presence of stray capacitance coupling between the primary and secondary transformer windings of the direct power supply to the amplifier results in a potential between the ungrounded chassis and ground which may approach the sum of the potentials developed across the primary and one of the secondary windings respectively. As one of the input electrodes is linked with the chassis of the amplifier, these capacitance introduced potentials result in a voltage drop from the patients body to ground which is much greater in magnitude than the voltage drop due to the antenna effect of the patients body alone so that the interference voltage appearing in the record is correspondingly greater.

Such difficulties as those pointed above are aggravated in portable instruments which are used in rooms of a hospital or clinic wherein it is often inconvenient or impossible to iind an object at ground potential to which to connect the grounding conductor.

Objects of this invention are to provide a circuit which substantially eliminates the interference resulting when the chassis of an amplifier or other electrical equipment is above groundpotential, which eliminates the danger of shock resulting from bodily contact with such chassis, which does not interfere with the normal operation of the equipment, which is automatic in operation, which warns the operator when the chassis is not grounded, and which is simple and economical to construct and install.

and at least one of the ends of the secondary or tertiary winding for supplying a counter electromotive force substantially equal in magnitude and opposite in phase to the voltage drop resulting from the stray capacitance coupling between the various windings of the transformer thereby eiectively maintaining the chassis at substantially ground potential.

In one specific aspect the counter electromotive force is obtained from the entire or a portion of the secondary winding of the transformer by coupling one end thereof to the primary winding terminal linked to the chassis by means of an impedance such as a capacitor, the characteristics of which impedance are such that the voltage impressed upon the impedance coupling the primary terminal to the chassis, is substantially equal in magnitude and opposite in phase to the voltage resulting from the stray capacitance coupling between the primary and secondary (or tertiary) windings so that the chassis is maintained substantially to ground potential.

In another specific aspect the counter electromotive force is tapped off a voltage dividing network including two potentiometers which are connected between the ends of the transformer secondary winding. The adjustable tap of one of the potentiometers is connected by means of an impedance, for example a capacitor, to the primary terminal coupled to the chassis. The other potentiometer tap is connected to the same terminal by a resistor so that there is a phase difference of approximately degrees between the voltages tapped from the respective potentiometers. By proper adjustment the sum of the voltages derived from the respective potentiometers is made equal in magnitude and opposite in phase to the voltage developed across the impenance coup-ing the primary terminal to the chassis as a result of the stray capacitance coupling oi' the transformer primary and secondary windings.

Another feature of the invention is the electrostatic shielding of the primary and secondary windings of the transformer so that the capacitance coupling and, therefore, the potential drop through the terminal coupling impedance which must be balanced by the counter electromotive force are reduced to a minimum.

A further feature is the use of a neon glow tube which connected in series with a resistor between the chassis and a body contact member which is located in such a position as to come in contact with the body or hands of the operator during normal operating procedure. If the ungrounded side of the power source is accidentally connected to the primary terminal which is coupled to the chassis, an electrical path is completed by the capacitance of the operators body to illuminate the glow tube thereby warning the operator of the high potential impedance impressed upon the amplifier chassis.

These and other objects, aspects and features of the invention will be apparent from and illustrated by the specific embodiment thereof now to be described with reference to drawings in which:

Fig. 1 is a circuit diagram of one embodiment of the inventionl incorporating an impedance by means of which the amplifier chassis is connected to one of the transformer primary terminals;

Fig. 2 is a circuit diagram of another embodiment ci the invention incorporating one means of obtaining a counter electromotive force;

Fig. 3 is a circuit diagram of a third embodiment of the invention incorporating other means of obtaining a counter electromotive force;

Fig. 4 is a circuit diagram of a fourth embodiment of the invention incorporating a half wave rectiiier and a transformer having a secondary winding which does not have a midetap; and

Fig. 5 is a circuit diagram of a fifth embodiment of the invention incorporating a transformer having a tertiary winding as a source of counter electromotive force.

The interference reducing circuit shown in Fig. 1 comprises a transformer T which is connected in the power supply section of an electrocardiograph in the conventional manner with the ends tl and t2 of the primary winding Tp linked with the terminals a and c which are adapted for connecting the primary winding with a single phase alternating power source of the conventional type having one grounded side. The mid-tap t of the secondary winding Ts is linked to the amplifier chassis A by the leads Il and l2. The ends t3 and t4 of the secondary winding Ts are connected by the wires i3 and I4 with the anodes pl and p2 respectively of a double diode vacuum tube rectifier V. The cathode lc of the tube V is coupled to the input of the amplifier filter section in the conventional manner.

Because of lthe physical proximity of the high voltage secondary winding to the primary winding, stray capacitance coupling exists between `the two. Furthermore, the coupling effect will, in general, be much greater between the primary and one end of the secondary than that which exists between the primary and the opposite end of the secondary. This asymmetry is shown dia- `grammatically by the single coupling capacitance Cs shown in broken lines interconnecting the winding end terminals tl and t3.

If the circuit described above is used with the amplifier chassis A insulated from ground. the stray capacitance Cs impresses upon the chassis A a potential to ground which is approximately equal in magnitude to the sum of the primary voltage and the voltage developed between the mid-tap t and the end t3 of the secondary winding.

To eliminate the inconvenience of a separate ground lead the mid-tap t and, therefore, the ampliner chassis A are connected to the primary winding end terminal t2 and therefore to the input terminal c by an impedance such as the series resistors Rl and R2. The value of each of the resistors is made approximately 0.1 megohm so that 200,000 ohms are in series between the chassis A and the terminal c. Such an impedance is low relative to the impedance of the stray capacitance coupling Cs thereby-effectively grounding the chassis if terminal cis the grounded side of the power supply. Under these conditions the chassis cannot inject a large interference Voltage into the patient circuit. If by accident the input terminal c is connected to the ungrounded side of they source, the 100,000 ohms impedance of either resistor introduces sufficient voltage drop to prevent a dangerous shock 4in case of simultaneous bodily contact with the chassis A and a grounded object. Two resistors are connected in series as a safety measure so that in the event of a breakdown of one resistor, the other resistor isolates the chassis from the power source.

Although the impedance of the series resistors RI and R2 is low as compared with the impedance of the stray capacitance coupling Cs, there is a1- ways a small potential drop across the resistors which raises the potential of the chassis `A slightly above ground potential thereby introducing some interference in the record. In Fig. 2 is shown acircuit which further reduces thepotential difference between the chassis A andground. This circuit is essentially similar to the `circuit shown in Fig. 1 and described in detail above, the transformer T, the tube V and theY resistors RI and R2 being interconnected in an analogous manner; but also includes a network for impressing a counter electromotive force across theresistors Rl and R2 which is substantiallyl equal in magnitude and opposite in phase to the voltage drop across the resistors resulting from the capacitance coupling Cs. i

rllhe last mentioned network comprises animpedance such as the resistor .R3 and a capacitor C connected in series between the primary winding end terminal t?? and the end tt. of the transformer secondary winding Ts. When the primary winding Tp is energized, the portion TS2 of the secondary winding Ts causes a 'current to flow through a series 4circuit vincluding the resistors tl' and the capacitor C and the resistor R3. The reacta-nce of the capacitor C is made very much greater than the ohmic impedance ef the resistor R0 so that theseries combination thereof acts essential-ly as aV simple capacitance circuit, the resistorA R being `used only as a safety device to prevent al low irnpe-dance connection between the chassis `and power supply through T82 if a breakdown or failure of the capacitor C occurs. With a transformer T of conventional design, a capacitor C with a reactance in the `range of 0.0001 mfg. `to 0.0006 mfg. has beenVV found` suitable. A resistoa` .v7 connected to the chassis A` The opposite secondary end terminal t/lis connected to the lter section by means of a half wave rectifier element X which may be, for'example a rectierr tube or a rectifying deviceY incorporating any` of the well known boundary layer elements. yThe secondary terminal t4 is connected to the end terminal t2 of the transformer primarywinding ATip by means of arseries circuit including the resistor R3and the capacitor C.

The function of the resistor R3 and thecapacitor of the embodiment in Fig. Il are analogousto that ofthe corresponding elements in the ernbodiment in Fig. 2. It will be noted in Fig. 4that it is the entire secondary winding Tls, rather than a portion thereof as is the case in Fig. 2, Vwhich Suppliesthe electromotive force, it thereforecbeingnecessary that the polarity `of the transformer windings be chosen whereby the* circulating current through they resistors Rl and R2 has sucha. phase relationship with respect to l the current resulting from the stray capacitance coupling that the currents oppose one another.

In the embodiment shown in Fig. 5, the counter electromotive force is supplied from'a'terytiary winding T2t wound upon the core of the transformer T2. The tertiary winding T215 is connected in series with the-resistors Rl, R2 and R3 `andthe capacitor-0. rihe common -junction of 4the capacitor Candthe resistor Ri is coupledto the terminal t2 of the transformer primary windingTZp. The common junction ofthe-resistor R2 and the'tertiary winding T2t is connected to the chassis A, The secondarywinding T25 `is connected to the full wave rectier tube in the `conventional manner, the mid-tap t being grounded and the end terminals t3 and t4 being linked with the'plates pi and p2 of the full wave rectier tubeeV by means lof the conductors I3 and I4v respectively. In this embodiment the resistors RI and R2 effectively ground the chassis A tothe power source, the small remaining voltage drop being neutralized by the counter electromotive force developed thereacross by the tertiary minal of said secondary winning withwone of said primary winding terminals, and second impedance connestingk the opposite terminal of said ndarywinding wit1 t" e primary wind- `ingterminal, the charac i, istics or" the second 'seing such and winding inal being selected so that avoltage is `ressed across they rst impedance which is ai "aligned-ual in magnitude and opposite `to the'voltage resulting from the stray capacitance coupling ioetween the primary and secondary windingsof the transformer thereby effectively` maintaining the chassis at substan- ,tially ground. potential.

,l end terminal connected tothe chassis of said amplifier anda primary winding, a terminal linked to each of -the respective ends lofthe primary Awinding for connecting said power source therewith,fazresistor coupling said end terminal of said secondary ,'windingwith one of said primary winding terminals, vand acapacitor connecting thel opposite end terminal of said secondary-winding with the same'primary winding terminal,- lthe reactivecharacteristics of the capacitorbeing suchend the secondary winding end terminal selected so that a voltage-is impressed across the resistor which is substantially-equal in magnitude and opposite in `phase to the'voltage resulting fromthe stray capacitance cou- .ig between the primary and secondary `windings of the transformer thereby effectively maintaining the chassis at substantially ground potential,

3. An interference'reducing circuit foren am- "i'ler energized from a grounded alternating power source and having a chassis as a common fuectrical return path, `said circuit comprising a former having a Secondaryr winding conn K'l to supply power to said amplifier, a' tertiary winding, and a primary winding having input iinals for/connecting said power source thereith, an impedance coupling the chasses with one of primary linput terminals, a second i* edance connecting one end of -the tertiary iding with the saine primary `input terminal, the opposite end of 'the tertiary winding being 4connected to the chassisthe characteristics of the second impedance being such, and the polarity of the tertiary winding being so'chosen that the current component through the rst impedance due to the tertiary voltage is equalv in magnitude and opposite in phase to the current component in the first impedance due to thev stray capacitance coupling between the windings of the transformer, thereby effectively maintaining the chassis at substantially ground potential,

4. An interference reducing circuit for an ampiier energized from a grounded alternating power source and having a chassis as a common electrical. return path, said ci cuit comprising a. transformer having a secondary winding connected to supply power to said amplier, a tertiary winding, and a primary winding having input terminals for connecting said power source therewith, a resistor coupling Ythe chassis with one of said primary input terminals, a capacitor connecting one end .of the tertiary winding. with the primaryinput terminal, the opposite` end of the tertiary winding being connected to ,the ch the reactive impedance of thel capacitor being such, and the polarity of tertiary Ywinding oeing so chosen that the current component through the resistor due to the tertiaryvoltage equal in magnitude and Opposite in phase to the current component in the lr ,i .tor dueto the stray capacitance coupling between the windings of the transformer, therebyeifectively maintaining the chassis at substantially ground potential.

ARTHUR -MILLER.

No references cited. 

